Aircraft: SMALL AIRCRAFT; HIGH WING; 1 ENG; FIXED GEAR
Phase of flight: Cruise
Anomaly: ATC Issue All Types; Inflight Event / Encounter Weather / Turbulence

Planned to depart O57 VFR. Surface winds were 190@6 and sky clear. Expected route follow Hwy 395 then direct. Preflight planning indicated area gusty winds in the valleys east of the sierra. Called an instructor at destination airport and we discussed local wind conditions; sky; visual to the east of the airport; and ride expectations over the Sierra with gusty winds on the east side of the Sierra. Overall; we generally agreed the ride would light to moderate turbulence on the east slope but smooth out once on west slope.Departed and climbed in valley surrounding the town to altitude of 12;500 before following Hwy 395. Observed cloud conditions over sierra confirmed my suspicion that route would be less turbulent south of TVL. Sacramento Valley was visible on other side of layers clouds over west slope of Sierra with strong winds out of the south. At the time; I believed it was possible to 'go over the top' at 14;500. Ride was smooth. As cloud tops rose I was unable to climb to stay in VFR and filed IFR with CTR and was given direct LIN @140. I told the controller that I wanted lower as able to descend below the cloud layer. Shortly after entering IMC the aircraft began to pick up ice. Prior to entering IMC; I had introduced carburetor heat & turned the pitot heat on. Soon after I advised ATC I was unable to maintain altitude and asked for lower. Acknowledging my predicament; ATC began vectoring my path while (presumably) checking area topography towards TVL. After approximately 2 to 3 minutes (guessing); I was able to see terrain to the left and below the aircraft; although I was still IMC. Shortly after that TVL came into view as well as Lake Tahoe off to the right of the aircraft and I diverted. Once clear of clouds and once again VFR; ATC provided local winds at TVL and I cancelled IFR and landed. On short final; I checked back with CTR and advised that we were short final; 'things' were okay; and thanked them. I rechecked surface winds and we landed. Once safely on the ramp; as we were securing the aircraft; a helicopter pilot appeared at the plane in a pickup truck and handed me a piece of paper with the telephone number and said the controller wanted to check on me and make sure that I had arrived safely. I called; a little later after gathering myself and we talked for about 7-10 minutes. I thanked the controller and told him I was very appreciative of his efforts. I was certainly glad he was there and that I had elected to file a clearance before entering IMC and at least had plan in place before things turned bad. We began to discuss the weird meteorological conditions and the noticeable temperature differential between [the east and west sides of the mountains]. Afterward; it brought into focus a similar remark made by the pilot about his observations from the ground while driving from his home. I told the controller of my flight planning efforts and that I generally had expected scattered to broken clouds over the eastern Sierra; as well as; about my direct call and discussion with folks at [my destination]. No other discussion occurred; nor was there mention of that any incident report would be filed; or any potential accusation regarding deviation from any FAR. When the call ended my impression was that the matter was closed and I was very fortunate; however; to be able to call and we both could learn from the experience. I felt compelled to file this report; however; because I had not experienced such a weather phenomenon in my 40 years and more than 4000 hours of safe flying and that I felt others could learn from what ultimately turned out to be judgment error. Once IMC and icing began I almost immediately had trouble maintaining altitude; my mind operationally went to: (1) 'stay calm'; (2) 'fly the airplane'; (3) 'listen to instruction' (as best possible); (4) respond to questions asked; and (5) continue to report your challenges to ATC. Thankfully; I believe that good training allowed me to laterthank ATC for their help and debrief the incident proactively. Later my passenger with me remarked on the calmness and professionalism he observed. As I write this account; more than 24 hours later; I am not sure my adrenalin level is totally back to normal; but I am certainly glad to be able to recount my experience for others. Most of all I remain thankful that I was able to call and personally thank the ATC controller and tell him that I wanted to communicate my lessons learned with others. Pilot transiting the Sierra Nevada Mountain Range at 12;000 feet encountered an overcast layer and was cleared VFR on top at 14;500 ft when clouds and icing were encountered. The pilot descended and diverted to TVL in order to clear the clouds.